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HAMAKER: NERVOUS SYSTEM OF NEREIS VIRENS. 97 
from the more peripheral part (Plate 2, Fig. 18). Its fibres are coarser 
and more compact, and they take a circular direction around the cord. 
Between the successive ganglia the fibres of this inner layer also pass 
in between the three longitudinal connectives. A few neuroglia nuclei 
lie scattered about among the nerve fibres of the cord. 
3. BRAIN. 
The brain of Nereis consists essentially of a central mass of interwoven 
fibres with a few irregular masses of neuropil, and a peripheral layer of 
cells loosely arranged in symmetrical groups. The cells have undergone 
a remarkable degree of differentiation, and the cells in each pair of groups 
have distinct characteristics. There are at least six distinct classes of 
cells ; while a number of the larger cells are arranged symmetrically in 
pairs, each pair having individual peculiarities of form and structure. 
The six classes, however, are not characterized by form and structure 
alone. Indeed, it is the difference in chemical reaction that is most 
distinctive, and calls for more than passing mention. The classes are 
as follows : — 
(1) In front, on each side of the brain, between the anterior median 
and the anterior lateral groups of nerves, there lie two masses of exceed- 
ingly small nuclei (Plate 3, Figs. 21, 24). The larger ventral mass is 
approximately crescentic in a transverse section of the brain with the 
concave side of the crescent lateral and partially embracing a mass of 
neuropil. The smaller, dorsal mass of cells is also crescentic in trans- 
verse section, but with the concave side toward the median plane. This 
crescent also partially embraces a mass of neuropil. In preparations 
stained with osmic acid or iron-hematoxylin the cells of these masses 
show only very faint traces of cell boundaries. The nuclei stain very 
deeply, and always contain a number of granules of various sizes. The 
nuclei are about 6 in diameter and are arranged in rows radiating from 
the neuropil (Fig. 21). They are set very closely in the rows, and are 
often almost in contact. The spaces between the rows are wider toward 
the neuropil, while toward the convex side of the mass the arrangement 
is more compact and the alignment of the nuclei almost disappears. 
The spaces between the rows of nuclei have a fine fibrous appearance, as 
though fibres from the cells passed out to the neuropil. Cells similar to 
these occur in other parts of the brain, e. g. a small group at the posterior 
border of the brain, just median to the root of the nerve which runs to 
the ciliated groove (XIII). Cells slightly larger, but otherwise like 
